(a) Field of the Invention
The present invention relates to a scanning type optical microscope, and more particularly to a photoelectric microscope using a position sensitive device.
(b) Description of the Prior Art
In scanning type optical microscopes, there has been proposed a method of obtaining a differential image by using a detector comprised of split detectors and by obtaining the difference in the signals derived from these split detectors, or more accurately, by dividing the difference signal by the sum signal. For example, in "Proceeding of SPIE", 1980, vol. 232, page 203, T. Wilson et al state that a differential image can be detected so on a scanning type optical microscope.
The principle thereof will be explained below. To begin with, a scanning type optical microscope is arranged so that, as shown in FIG. 1, a light coming from a spot light source 1 such as a laser is collected by an objective lens 2 to illuminate a sample 3, and the light which has been transmitted through the sample 3 is detected by a detector 4. Along therewith the sample 3 is subjected to a raster-scanning in a direction normal to the optical axis, and, in synchronism therewith the detection signal is imaged on a television screen, whereby a microscopic image of the sample 3 can be observed. When compared with the conventional microscopes, the scanning type optical microscope illuminates the sample 3 by a spot light, so that flare or like phenomenons are eliminated, and as a result a substantially improved image is obtained.
In such a case, by constructing the detector 4 with two detectors 5 and 6, and by using a difference signal of the outputs from the respective detectors, there is obtained a differential image.
Here, description will be made of the logic of formation of a differential phase image.
For the sake of simplicity, a one-dimensional image is considered. The intensity of an image due to a partial coherent focusing is indicated, in general, as follows: ##EQU1## wherein: T(m) represents the Fourier conversion of the transmittivity of an object; and
C(m;p) corresponds to the transfer function of the optical system. PA1 .lambda. represents the wavelength of the light.
When the sensitivity of the detectors is assumed to be D(.xi.), and when the pupil function of the optical system is assumed to be P(.xi.), then C(m;p) is indicated by ##EQU2## wherein: f represents the focal distance of the system; and
Here, an object having a weak contrast is considered. In such a case, it is only necessary to consider C(m;0). When the difference between the signals is considered, assuming that D(.xi.) is the sensitivity of the split detectors, C(m;0) will assume such a pattern as shown in FIG. 2. The fact that the transfer function takes such a pattern as this indicates that the differentiation of the image is obtained. Also, when the sum signal is employed, there is obtained an ordinary image. As stated above, only by the change in the mode whether the difference between the signals is used, or the sum of the signals is used, is there the feature that either a differential image or an ordinary image is obtained.
Here, split detectors 7 and 8 whose sensitivity distribution D(x) assumes a wedge form as shown in FIG. 3 will be considered. Reference numeral 9 represents an optical axis. When the difference in the signals delivered from these detectors is employed, C(m;0) becomes as shown in FIG. 4, and a differential image which is superior to that obtained in the case of FIG. 2 is provided. In case of using a sum signal, however, there is the drawback that the image becomes one with stressed high frequency components.
As stated above, the conventional art has employed split type detectors, which, accordingly, had the drawback that when it was intended to obtain an excellent differential image, there could not be obtained an excellent ordinary image. Also, the center of the interface of the split detectors had to be aligned with the optical axis, and thus an adjustment therefor was required. Also, in order to modify the sensitivity of the detector into a wedge form, there was the necessity, in practice, to use a filter or like means, with the result that the amount of light could not be utilized efficiently. Furthermore, when assembling actual circuitry, two amplifiers were required, which gave rise to the problem that the device became costly.